A circuit arrangement for the switching-over of a bistable relay with the aid of a semiconductor is, for example, known from the book "Relais Lexikon" (Relay Lexicon) by H. Sauer, first edition, 1975, page 12. Upon application of the excitation voltage, a first transistor connected in series with the coil and capacitor is conductive, the relay actuates and the capacitor is charged. If a positive control signal is provided at the input of a second transistor, then the first transistor is blocked and a third transistor, connected in parallel to the coil and capacitor, conducts. The capacitor is discharged through this third transistor and the relay switches back. If the control signal jumps to zero value, then the second and third transistors are again blocked, the first transistor is conductive and the capacitor is again charged, with which the relay switches over.
A circuit arrangement of this type is expedient for this operation of bistable relays when the polarity of the excitation voltage remains unchanged. The relay remains in its switched position after charging of the capacitor, independent of whether the excitation voltage is switched off or is applied as before. A diode connected in series prevents a slow discharge of the capacitor when the excitation voltage is absent. The relay is switched back only by a positive control pulse at the input of the second transistor. When this pulse cannot be produced from the excitation voltage, such as when the excitation voltage is switched off, the necessity for an external control signal source results.
In addition to this, a circuit arrangement for the controlling of a bistable relay is known from German Published Patent Application No. 2 624 913, in which this this circuit arrangement acts as a monostable relay and thus switches back automatically to the starting position upon insufficiency of the excitation voltage. This is attained in that an evaluation circuit fed with the excitation voltage is connected, on the one hand, and the control electrode of the semiconductor, on the other hand, the evaluation circuit blocking the semiconductor when the excitation voltage is present and rendering the semiconductor conductive when such voltage is absent. For the prevention of an unintended discharge of the capacitor, a diode is also connected in the current path. A series resistance in the same path serves for short-circuit-proofing of the semiconductor as well as for corresponding dimensioning for the realization of a defined voltage drop, with which a relay with economically-fabricatable low-voltage windings may be operated with higher voltages such as, for example, line voltage. With this, however, an evaluation circuit required to achieve the monostable switch operation of the relay leads to a relatively high expenditure of components.